Providing a good seal between the piston and cylinder is especially important in so-called “free-floating” piston compressors. In a free-floating piston compressor there is no piston rod and the piston movement is governed by the differential pressure between chambers within the compressor cylinder on opposite sides of the piston. One chamber acts as the drive chamber and the opposite chamber acts as the compression chamber. During a compression stroke, hydraulic fluid is supplied to the drive chamber to compress the fluid in the compression chamber. During an intake stroke, hydraulic fluid is removed from the drive chamber and new fluid flows into the compression chamber.
Although the actual differential pressure can be small, the absolute fluid pressures can be very high. For example, free-floating piston compressors can have a discharge pressure in excess of 5000 psi (about 34.5 Mpa).
The failure of the piston ring seal can have damaging consequences. With a closed loop hydraulic system it is important to prevent leakage of the hydraulic fluid out of the hydraulic system. Leakage of the hydraulic fluid into the compression chamber would also contaminate the compression fluid. By way of example, if the compression fluid is a gas that is used as fuel for an engine, and hydraulic fluid leaks into the fuel, the presence of hydraulic fluid in the fuel can cause higher pollutant levels in the engine exhaust. A fluid separator may be required downstream of the compressor if the piston seal cannot provide a reliable and effective seal. In addition to the added complexity and cost of adding a fluid separator, the pressure drop across such a device reduces the efficiency of the compressor system. Accordingly, a piston seal that minimizes leakage of hydraulic fluid into the compression fluid is preferable.
In addition, leakage of compression fluid into the hydraulic system must be guarded against. For example, if the compression fluid is a gas, introduction of gas into the hydraulic system could cause damage to the hydraulic pump or could otherwise cause the hydraulic system to cease functioning.
Accordingly, there is a need for a method and apparatus for a high pressure reciprocating piston and cylinder arrangement that scavenges fluid that leaks by the piston seals and returns the scavenged fluid to the piston chamber from which it originated.